Diffractive optics using optical elements that affect propagating wavefronts by means of diffraction are known. Exemplary diffractive optical element (DOE) structures are diffraction gratings, zone plate lens and holographic mirrors. DOEs in which the diffracting element sizes are approaching or approximately equal to the wavelength of light are generally known as holographic optical elements (HOEs). One advantage to diffractive optics is that a structure, such as a DOE lens, may be constructed on a flat surface and can thus be smaller, cheaper and more easily aligned than a refractive optics counterpart. One disadvantage is that because diffractive optics structures are formed of patterns of diffracting elements they are sensitive to the wavelength of the light used.
HOEs can be recorded on optical media, such as photographic films, to create optical devices like lenses and prisms. The hologram patterns are diffraction patterns established by recording the interference pattern of two laser beams. The resulting diffraction pattern has constituent elements with dimensions on the order of a wavelength. HOEs can also be produced by mechanical means such as engraving with a diamond tool, photolithography or embossing with a hard metal master.
HOEs are advantageous, in certain applications, for a number of reasons. HOEs may be quite thin in profile, thereby allowing the fabrication of numerous optical elements of smaller size than traditional optical counterparts. Further, as HOEs are planar devices, complex optical systems may be assembled in a simplified manner using less space than typical multi-element optical systems. In fact, HOEs may be self-positioning, thereby greatly reducing the alignment problems associated with optical systems, especially complex optical systems.
In general, the diffraction pattern of HOEs are designed to transmit incident light into modes, or directions. Modes are conventionally labelled m=0+1, −1, +2, −2, etc. . . . according to their location with respect to the incident light. If the HOE is to be used as a lens or mirror two primary modes are typically involved, the m=0 mode and the m=−1 mode. In the m=0 mode, incident light appears unaffected by the HOE, i.e., if the HOE is a reflective element, light will be reflected into the zero order mode as though the light had been reflected by a flat mirror surface, and if the HOE is a transmissive element, light will exit the element as if it had been transmitted through a transparent optical media. The m=−1 mode is a direct result of the designed optical function of the HOE. This mode will be generally offset from the m=0 mode. In typical devices, the HOE is chosen so that the amplitude of light in the m=0 mode is minimized through destructive interference, and the amplitude of the desired m=−1 mode is maximized through constructive interference. The angle of incident light and size of the diffractive elements is generally chosen so that other modes that could interfere with the desired optical performance do not exist.
With their ability to reflect light from a normal path (i.e., coinciding with a m=0 mode) into a reflected mode (m=−1), there is a desire to employ HOEs in switching devices. Current designs of HOEs limit the use of HOEs as optical switches. HOEs are generally formed either within the bulk of a material or on the surface of a material. Holograms recorded in the volume of a holographic material have low losses, but are very difficult to mass-produce. Examples are three-dimensional structures formed in volume using electro-holographic materials switchable by application of an electric field. On the other hand, surface relief holograms may be mass-produced, but suffer from low optical efficiency. An added problem with these surface-relief holograms is that they are not switchable.
Despite the above shortcomings of existing HOEs and DOEs, it is nonetheless desirable to use HOEs and DOEs as switches with optical media.